The traditional implantable cardiac pacemaker includes a pulse generator device to which one or more flexible elongate lead wires are coupled. The device is typically implanted in a subcutaneous pocket, remote from the heart, and each of the one or more lead wires extends therefrom to a corresponding electrode, coupled thereto and positioned at a pacing site, either endocardial or epicardial. Mechanical complications and/or MRI compatibility issues, which are sometimes associated with elongate lead wires and well known to those skilled in the art, have motivated the development of cardiac pacing devices that are wholly contained within a relatively compact package for implant in close proximity to the pacing site, for example, within the right ventricle (RV) of the heart. With reference to FIG. 1, such a device 100 is illustrated, wherein pace/sense electrodes 111, 112 are formed on an exterior surface of a shell 101 that hermetically contains a pulse generator including pulse generator electronics and a power source. Shell 101 is preferably formed from a biocompatible and biostable metal such as titanium overlaid with an insulative layer, for example, medical grade polyurethane or silicone, except where electrode 112 is formed as an exposed portion of capsule 101. A hermetic feedthrough assembly (not shown), such as any known to those skilled in the art, couples electrode 111 to the pulse generator contained within shell 101.
FIG. 1 further illustrates a fixation member 115 mounted to an end of shell 101, in proximity to electrode 111, in order to fix, or secure electrode 111 against the endocardial surface in the apex of the RV. However, in some patients, pacing stimulation may be more effective if delivered to a right atrial site or a left ventricular site. Thus, alternative forms of relatively compact pacing devices have been developed for these alternate implant sites and there is a need for corresponding implant systems and methods.